The invention relates to charge transfer circuits such as those employed in "skimming" a charge signal. It particularly relates to a method for achieving uniform background charge subtraction in such circuits.
U.S. Pat. No. 3,845,295 to W. F. Kosonocky (one of the present inventors) and B. F. Williams for "Charge Coupled Radiation Sensing Circuit with Charge Skim-Off and Reset" issued Oct. 29, 1974, describes a charge skimming circuit and discusses also where such circuits are useful. In the patented circuit, a photodiode or other radiation sensor accumulates charge when a radiation image is projected thereon. After a suitable integration time, that portion of the charge which is greater than a given level is passed through the conduction path of one field effect transistor (FET) to a stage of a CCD register. Then, the remainder of the charge signal is passed through the conduction path of a second FET transistor to a drain, resetting the radiation sensor in the process. If, as is often the case, the threshold voltage of the FET's varies from one location to another, there will be non-uniformities introduced into the skimming process. For example, if the threshold voltage for the various reset transistors is not the same, the radiation sensors in the array will not be reset to the same reference voltage level. Similarly, if the transistors employed to skim the charge signals do not have the same threshold voltage, the point at which the skimming starts will not be the same from one radiation sensor to another. Also, if the threshold level of a reset transistor for a location differs from that of the skimming transistor of heat location, the difference between threshold levels will be introduced into the skimmed signal as a charge increment (or as a charge deficit). In all of these cases, the differences in skimming and reset levels are manifested as fixed pattern noise in the reproduced image.
U.S. Pat. No. 4,040,076 to W. F. Kosonocky and E. S. Kohn for "Charge Transfer Skimming and Reset Circuit" issued Aug. 2, 1977 discloses a circuit of the same general type discussed above in which non-uniformities due to different threshold voltages are essentially eliminated. In the circuit described in this patent the portion which exceeds a given level, of a charge signal present at a radiation sensor, is passed to a storage site via the conduction path beneath a gate electrode. The remainder of the charge signal is removed to a drain through the same conduction path, the radiation sensor being reset to a reference level in the process. The use of a common conduction path for setting both the charge signal "skimming" level and the reset level, improves the uniformity of the skimming process from one sensor to another. Although this circuit largely solves the problems enumerated above it can be limited in its application. That is, in operation, the circuit allows the removal of a fixed amount of charge from the charge detected by each sensor element only once for each frame time or integration time. If, however, the total detected charge during one integration should be considerably larger than the capacity of the detector well the system would saturate and fail. This is complicated by the fact that the fabrication of radiation sensor/CCD register whose detector capacity is much larger than the register well capacity can be difficult to achieve. Consequently, in order for background removal to be used advantageously when these two capacities are comparable it is necessary to remove charge from the detectors several times during the integration period.
The present invention comprehends a method of operation that is applicable to circuits of the type described that meets this multiple charge removal per integration period requirement.